The pretext is that his travel to another state was assumed to be to buy weed. They appear to be relying on his existing warrant and criminal record to justify having used Flock data to not just locate him, but manufacture probable cause.

Unless they have direct evidence (photo from Flock of him ripping bong hits in his car at a light) prior to the stop to attach probable cause to, it's a pre-crime judgment.

IANAL (if you are, feel free to poke holes), but I wouldn't presume it unlikely he walks under the 4th amendment, even if it's through appeals.